A REVIEW OF THE RHEOLOGICAL EFFECTS OF POWER LAW DRILLING FLUIDS ON CUTTINGS TRANSPORTATION IN NON-VERTICAL BOREHOLES
Cuttings transportation during in non-vertical boreholes is necessary for oil and gas wells. Adequate cuttings removal from a well in drilling is critical for cost-effective drilling as high annular cuttings buildup often leads to high risk of stuck pipe, reduced rate of penetration and other impediments to standard drilling and completion procedures. This study investigates how rheological parameters influence the removal of cuttings in non-vertical boreholes. It contributes to work already done to ensure efficient hole cleaning process. In this study, the rheological parameters examined were the flow index (n), consistency index (K), plastic viscosity (PV), mud yield point (YP), YP/PV ratio, apparent viscosity and effective viscosity. Fifteen mud samples, three annular velocities (3.82, 2.86 and 1.91 ft/sec) and three hole angles (30o, 45o and 70o) were considered. An Excel Spreadsheets program was used to determine the parameters. The results of this study show that, higher annular mud velocities are required for efficient hole cleaning in directional wells than in vertical wells. Increasing values of YP, YP/PV ratio and K promote effective cuttings transport while the value of n should be low. Effective and apparent viscosities also should be high.
1.1 Problem Definition
Many materials of engineering interest must be handled and transported as slurries or suspensions of insoluble particulate matter. Transportation of cuttings in non-vertical boreholes is of no exception. Almost the same thing occurs whereby the cuttings act as the solids in the drilling fluid. In spite of the many technological advances that have accompanied the drilling of non-vertical boreholes, one significant remaining challenge is effective cuttings transport, particularly in deviated wells. The transportation of cuttings during drilling has a major influence on the economics of the drilling process. Problems that can occur as a result of inefficient hole cleaning from cuttings include reduced weight on bit, increase risk of pipe stuck and inability to attain the desired reach, reduced rate of penetration (ROP), extra cost because of the need of special additives in the drilling fluid, extra pipe wear, transient hole blockage which can lead to lost circulation and wasted time for wiper tripping. These problems have prompted significant research into cuttings transport during the past 50 years. (Kelessidis, 2004).
Hole cleaning relying on viscous fluids in laminar flow for drilling has proved to be inefficient because of the inability to rotate the string to agitate bedded cuttings. Alternatively, a high fluid flow to induce turbulent flow regime is more effective for hole cleaning, but difficult to achieve because of high friction pressures in the drillpipe. Therefore a bed of cuttings is almost always present in non-vertical boreholes. For laminar flow, the distance that a particle will travel (downstream) before it falls across the annulus clearance can be calculated using Stokes’ law and the local viscosity while flowing can also be calculated. This analysis may be easily applied to optimize mud selection and wiper trips. Applying this model to high low-shear rate-viscosity (LSRV) gels shows that they may perform well inside casing but are expected to do a poor job of hole cleaning in a narrow openhole horizontal annulus without rotation.
For turbulent flow in horizontal wells, the concept of using annular velocity (AV) as a measure of hole cleaning is insufficient. A more complete term called AVRD is introduced, which is the product of the AV and the square root of the hydraulic diameter. This term can be used to compare cuttings transport in turbulent flow in horizontal wells of different cross sectional areas. (Leising et al., 1998). Rheology which is the study of the flow and deformation of fluids is an important contributing factor to these problems. Rheology describes the relationships between shear rate and shear stress. Pilehvari, Azar, and Sanchez2,16 state that fluid velocities should be maximized to achieve turbulent flow, and mud rheology should be optimized to enhance turbulence in inclined/horizontal sections of the well bore. The purpose of this study is to investigate how rheological parameters influence the removal of cuttings in non-vertical boreholes.
The objectives of this research are:
· To present a review of cuttings transport in vertical, directional/horizontal well bores.
· To provide a critical review of how rheology affects cuttings transportation in non-vertical boreholes.
· To identify the critical parameters that affect effective removal of cuttings in the drilling of non-vertical boreholes.
· To propose a methodology for analysing the rheological parameters that affect cuttings transportation in non-vertical boreholes.